This invention relates generally to the generation and confinement of an energetic plasma by magnetic fields and is particularly directed to the initiation and sustaining of a fusion plasma in a sheromak reactor.
A spheremak reactor uses toroidal and poloidal magnetic fields to confine a doughnut-shaped plasma. In a speheromak, the currents generation the toroidal field within the plasma itself, eliminating the large external toroidal field coils required in tokamak reactors. The spheromak geometry therefore provides various advantages over that of the tokamak such as improved access to the plasma for placement of thermal conversion blankets, the creation of plasmas with low aspect ratios and increased stability, a high current density minimizing the requirement for auxiliary heating, and a high beta value representing the ratio of plasm pressure to magnetic field pressure.
The sustainment of the spheromak configuration is highly desirable for assisting in the attainment of fusion conditions from an initial discharge or for maintaining a steady-state fusion plasma. Sustainment is used to describe any method of actively driving plasma currents to either extend the discharge lifetime, maintain a steady-state discharge, or increase plasma currents after the formation phase. Steady-state or long-pulse (wherein the discharge time is much longer than a resistive decay time) operation has many advantages over pulsed operation including a reduction in mechanical fatigue due to cyclic magnetic and thermal stresses an an increase in energy efficiency by eliminating energy losses incurred during start-up of a discharge. Near term advantages of sustainment would be derived from the providing of a long lived, time-independent plasma so that the confinement properties of the spheromak could be easily studied.
Various techniques have been proposed for the sustainment of spheromaks. These proposals, which have met with only limited degrees of success, include the merging of spheromaks, the application of oscillating field current drive (OFCD) using audio frequencies, radio frequency current drive, the application of direct current (DC) from electrodes, the Rotamak concept, and neutral as well as charged particle beam injection. Present Alfven electrode gun schemes have the advantage of a simple operation requirement that permits the plasma to be easily translated into the experimental area. In addition, the DC electrode gun approach has sustained spheromaks experimentally for much longer than a resistive decay time. However, this approach requires a substantial amount of electrode discharge to create the toroidal magnetic field and to propel the plasma through the electrode gun muzzle against poloidal field pressure. Furthermore, the possible advantage of inductive sustainment over DC current drive using electrodes is a potential reduction if impurity influx due to the elimination of a material surface, in contact with the plasma, through which large currents are drawn. Inductive sustainment is extendable to long time cycles and may be able to maintain a discharge indefinitely.
U.S. Pat. Nos. 4,363,776 to Yamada et al and 4,436,691 to Jardin et al, both assigned to the assignee of the present application, disclose inductive approaches to the formation and sustainment of a spheromak plasma. However, because inductively produced spheromak plasmas are subject to resistive decay and because heretofore the poloidal and toroidal fields have not been sustainable as both are produced in part or in whole, respectively, by plasma currents, the aforementioned inductive approaches have been used only in a pulsed operation, i.e., the method is repeated at regular intervals, and have not been suitable for continuous, or steady-state, operation.
The aforementioned, cross-referenced patent application also discloses an inductive approach to the sustainment of a spheromak plasma involving the initiating of a plasma discharge by means of the combination of poloidal and toroidal magnetic fields in an evacuated vacuum vessel containing a neutral species, wherein the poloidal magnetic field is comprised of first and second component poloidal magnetic fields of different strength. The thus produced plasma is allowed to expand in the direction of the weaker poloidal magnetic field, with a portion of the expanded plasma pinched off so as to produce a line-linked spheromak plasma partially connected to the toroidally shaped flux core within the vacuum vessel. The poloidal and toroidal magnetic fields .psi. and .phi. are then subject to oscillation such that .psi. and .phi. have different phases, where preferably the poloidal and magnetic fields are 90.degree. out of phase. The plasma may be partially pinched off by either energizing a set of pinching coils or by simultaneously reversing the directions of the poloidal and toroidal currents in the flux core.
The present invention is directed to an arrangement for the inductive generation and sustainment of a spheromak plasma involving the use of a poloidal flux-amplifying inductive transformer aligned along the major axis of a flux core and comprised of a solenoidal coil. Reversal of the current in the solenoidal coil results in a poloidal flux swing and the conversion of a portion of the poloidal flux to toroidal flux for either plasma generation or sustainment.